The plasma membrane and cytoplasmic membranes of plants, like those of animal cells, are composed of lipids and proteins that are often glycosylated. Likewise, the composition from one membrane type to another is highly heterogeneous. There is some evidence to suggest that the composition, particularly of the lipid component, may change in response to environmental conditions such as temperature, water stress, etc. as well as during growth, development and ultimately senescence of the cell. It is believed that these changes are required to adjust the physical characteristics of membrane structures so that they may perform their necessary physiological tasks when environmental factors change. If the environmental conditions are altered beyond the normal limits within which the plant survives, the cell membranes are often found to undergo gross structural changes. These structural perturbations include phase separation of the membrane constituents and are associated with characteristic disturbances of function such as loss of selective permeability and transport processes. In most instances, the observed phase separations appear to be driven by phase changes in the membrane lipids. Some lipids extracted from algae and plant membranes are known to exist in a bilayer gel phase when dispersed in aqueous systems at the growth temperature while other lipid fractions are in a liquid-crystalline state. Nearly all membranes contain varying proportions of their lipid complement that do not form bilayer structures under such conditions and most commonly adopt an hexagonal-II arrangement. In this review the importance of lipid phase behaviour is discussed in the context of membrane stability at different temperatures.